The INES manual is pretty clear that every release above a certain point is by definition level 7:
2.2.2. Definition of levels based on activity released2
Level 7
“An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of more than several tens of thousands of terabecquerels of 131I.”
This corresponds to a large fraction of the core inventory of a power reactor, typically involving a mixture of short and long lived radionuclides. With such a release, stochastic health effects over a wide area, perhaps involving more than one country, are expected, and there is a possibility of deterministic health effects. Long-term environmental consequences are also likely, and it is very likely that protective action such as sheltering and evacuation will be judged necessary to prevent or limit health effects on members of the public.
Remember that the whole purpose of the INES rating system is to manage communications with the general public about the significance of an incident. As pointed out in post 8, the whole "pyramid" is a rather peculiar way of expressing this, since its visual representation emphasizes frequency over consequences. Given the rhetorical purposes of the INES scale, I see no particular reason why those who set it up would feel remotely compelled to revamp the scale in the face of a "worse than as bad as it can get" accident.
The notion of a logarithmic scale is meant to condition media reporting rather than be taken too literally as a quantitative scale. The Richter scale needs no recalibration in the face of an unusually large earthquake, because it has a very specific physics basis; the INES rating scale does not.
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